1. Field of the Invention
The present invention relates in general to active vibration control. Although there are methods of active vibration control in the time domain, the present invention is particularly related to a methodology of active vibration control in the frequency domain. The present invention relates generally to a method and apparatus for isolating mechanical vibrations in a structure or body which is subject to harmonic or oscillating displacements or forces, and is of particular utility in the field of aircraft, in particular, helicopters and other rotary wing aircraft.
2. Description of Related Art
For many years, effort has been directed toward the design of apparatus for isolating a vibrating body from transmitting its vibrations to another body. Such apparatus are useful in a variety of technical fields in which it is desirable to isolate the vibration of an oscillating or vibrating device, such as an engine, from the remainder of the structure. Typical vibration isolation and attenuation devices (xe2x80x9cisolatorsxe2x80x9d) employ various combinations of the mechanical system elements (springs and mass) to adjust the frequency response characteristics of the overall system to achieve acceptable levels of vibration in the structures of interest in the system. One field in which these isolators find a great deal of use is in aircraft, wherein vibration-isolation systems are utilized to isolate the fuselage or other portions of an aircraft from mechanical vibrations, such as harmonic vibrations, which are associated with the propulsion system, and which arise from the engine, transmission, and propellers or rotors of the aircraft.
Vibration isolators are distinguishable from dampening devices in the prior art that are erroneously referred to as xe2x80x9cisolators.xe2x80x9d A simple force equation for vibration is set forth as follows:
F=m{umlaut over (x)}+c{dot over (x)}+kx
A true vibration isolator utilizes acceleration of a fluid body m{umlaut over (x)} to cancel the displacement of vibration kx. On the other hand, a dampening device is concerned with restricting flow of a fluid or other body, and thus velocity c{dot over (x)} and does not cancel vibration, but merely absorbs its frequency.
One important engineering objective during the design of an aircraft vibration-isolation system is to minimize the length, weight, and overall size including cross-section of the isolation device. This is a primary objective of all engineering efforts relating to aircraft. It is especially important in the design and manufacture of helicopters and other rotary wing aircraft, such as tilt rotor aircraft, which are required to hover against the dead weight of the craft, and which are, thus, somewhat constrained in their payload in comparison with fixed-wing aircraft.
Another important engineering objective during the design of vibration-isolation systems is the conservation of the engineering resources that have been expended in the design of other aspects of the aircraft or in the vibration-isolation system. In other words, it is an important industry objective to make incremental improvements in the performance of vibration isolation systems which do not require radical re-engineering or complete redesign of all of the components which are present in the existing vibration-isolation systems.
A marked departure in the field of vibration isolation, particularly as applied to aircraft and helicopters is disclosed in commonly assigned U.S. Pat. No. 4,236,607, titled xe2x80x9cVibration Suppression System,xe2x80x9d issued Dec. 2, 1980, to Halwes, et al. (Halwes ""607). Halwes ""607 is incorporated herein by reference. Halwes ""607 discloses a vibration isolator in which a dense, low-viscosity fluid is used as the xe2x80x9ctuningxe2x80x9d mass to counterbalance, or cancel, oscillating forces transmitted through the isolator. This isolator employs the principle that the acceleration of an oscillating mass is 180xc2x0 out of phase with its displacement.
In Halwes ""607, it was recognized that the inertial characteristics of a dense, low-viscosity fluid, combined with a hydraulic advantage resulting from a piston arrangement, could harness the out-of-phase acceleration to generate counter-balancing forces to attenuate or cancel vibration. Halwes ""607 provided a much more compact, reliable, and efficient isolator than was provided in the prior art. The original dense, low-viscosity fluid contemplated by Halwes ""607 was mercury, which is toxic and highly corrosive.
Since Halwes"" early invention, much of the effort in this area has been directed toward replacing mercury as a fluid or to varying the dynamic response of a single isolator to attenuate differing vibration modes. An example of the latter is found in commonly assigned U.S. Pat. No. 5,439,082, titled xe2x80x9cHydraulic Inertial Vibration Isolator,xe2x80x9d issued Aug. 8, 1995, to McKeown, et al. (McKeown ""082). McKeown ""082 is incorporated herein by reference.
Several factors affect the performance and characteristics of the Halwes-type isolator, including the density and viscosity of the fluid employed, the relative dimensions of components of the isolator, and the like. One improvement in the design of such isolators is disclosed in commonly assigned U.S. Pat. No. 6,009,983, titled xe2x80x9cMethod and Apparatus for Improved Isolation,xe2x80x9d issued Jan. 4, 2000, to Stamps et al. (Stamps ""983). In Stamps ""983, a compound radius at the each end of the tuning passage was employed to provide a marked improvement in the performance of the isolator. Stamps ""983 is incorporated herein by reference.
Although the foregoing inventions represent great strides in the area of vibration isolation, certain shortcomings remain, in particular, the ability to actively tune the isolator.
Therefore, it is an object of the present invention to provide a vibration isolation system in which the isolator can be actively tuned.
It is another object of the present invention to provide a vibration isolator that allows active tuning of the isolator, as well as, simultaneous vibration treatment of multiple harmonics.
It is yet another object of the present invention to provide a vibration isolator that allows active tuning of the isolator, as well as, active xe2x80x9cnegativexe2x80x9d damping which results in near zero vibration transmissibility.
These and other objectives are achieved by providing a tunable vibration isolator with active tuning elements having a housing which defines fluid chambers. A piston is disposed within the housing. A vibration isolation fluid is disposed within the fluid chambers. A passage having a predetermined diameter extends through the piston to permit the vibration isolation fluid to flow from one fluid chamber to the other. The tunable vibration isolator may employ either a solid tuning mass approach or a liquid tuning mass approach. In either case, active tuning elements, or actuators, are disposed in the fluid chambers to selectively alter the dynamic characteristics of the vibration isolator.
Preferably, the relatively enlarged portion is defined by a compound radius which extends over a predetermined length of the passage.
Additional objectives, features and advantages will be apparent in the written description which follows.